Typically, compound bows are configured so that a bowstring is easily pulled without using a large force and arrow shooting power is increased during shooting, by using an effect of a cam or wheel, to thus result in a fast speed of an arrow and have very strong power, and are widely used mainly for hunting.
When users having different arm's reaches use an identical compound bow, it is necessary to adjust a draw length of a bowstring in a let-off state, that is, in the latest state immediately before an arrow is launched. Otherwise, when a user is skilled in comparison with a beginner time, there may be the need to increase the draw length of the bowstring. A structure of a compound bow (Korean Patent No. 10-1418312, published on Jul. 10, 2014) proposed by the present applicant in order to easily adjust the pulling length of the bowstring in the let-off state is shown in FIGS. 1-3.
FIG. 1 is a perspective view of a conventional compound bow. FIG. 2 is a plan view of a pulley assembly coupled to a lower limb in FIG. 1. FIG. 3 is a plan view showing a pivoted state of a cam module in the compound bow of FIG. 1.
Referring to FIGS. 1 to 3, a compound bow includes: a bow main body 100 including a pair of limbs 103 that are respectively coupled to both ends of a handle 102; upper and lower pulley assemblies 107 and 108 that are respectively coupled to the rear end of each limb 103; a bowstring 140; cam cables 150 that are wound around a cam 200 of each of the upper and lower pulley assemblies as the bowstring 140 is pulled.
The bow main body 100 includes a handle 102 at a central portion of which a grip portion is formed so as to be gripped by a user, and a pair of limbs 103 that are respectively coupled to both ends of the handle 102 in which two branches are formed at the rear portion of each limb 103. A rotating shaft 101 is formed at the rear end of each limb 103, in which a pulley assembly is rotatably coupled on the rotating shaft 101 between the two branches at the rear end of each limb 103.
Each of the upper and lower pulley assemblies is rotatably coupled to the rotating shaft 101 at the rear end of each limb 103, and includes: a pulley 110 that is rotatably coupled to the rotating shaft 101 formed at the rear end of each limb 103; and a cam 200 coupled to one side of the pulley 110 and rotating with the pulley 110.
Each pulley 110 is formed of an oval-like plate-shaped member, and has an eccentric through-hole that is formed at the center of the pulley 110 and through which the rotating shaft 101 is coupled. A fixing protrusion 111 for fixing one end of the bowstring 140 wound on the guide groove is formed at one side of each pulley 110. In addition, fixing protrusions 112 and 113 are formed in each pulley 110 in which the cam cables 150 are fixed to the fixing protrusions 112 and 113, respectively. The bowstring 140 is wound in the guide groove of the pulley 110 of each pulley assembly and thus both ends of the bowstring 140 are coupled to the fixing protrusions 111 formed on the respective pulleys 110.
The cam cables 150 are formed between a pair of the limbs 103 of the bow main body 100 and are wound on the cams 200 formed in the respective pulleys 110, as the bowstring 140 is pulled. One end of each of the cam cables 150 is coupled to the fixing protrusion 113 formed on the pulley 110 of one of the pulley assemblies, and then is wound around the rotating wheel 120 that is rotatably coupled to the rotating shaft 101, to then be extended toward the other one of the pulley assemblies, and the other end of each of the cam cables 150 is fixed to the fixing protrusion 112 of the pulley 110 of the other one of the pulley assemblies. Therefore, as the bowstring 140 is pulled, the cam cables 150 are wound on the cam 200 that is coupled to the other pulley 110 of the pulley assemblies.
The cam 200 is formed in each pulley 110 and is rotated with rotation of the pulley 110, and includes: a cam cable support portion 210 fabricated in an arc-shaped form and on which one of the cam cables 150 is wound; and a cam module 220 that has a pivot shaft 221 at a position spaced by a predetermined distance from the rotating shaft 101 of the pulley 110 to which the cam 200 is coupled, and that is rotatably coupled to the pivot shaft 221 by a predetermined angle from the cam cable winding portion 210, in which a cam cable winding groove is formed on the outer circumferential surface of the cam module 220, and the one of the cam cables 150 is wound around the cam cable support portion 210 and then sequentially wound on the outer circumferential surface of the cam module 220 when the bowstring 140 is pulled.
The cam module 220 has the pivot shaft 221 at a position spaced by a predetermined distance from the rotating shaft 101 of the pulley 110 to which the cam 200 is coupled, and is rotatably coupled to the pivot shaft 221 by a predetermined angle from the cam cable support portion 210, and is configured to have a gentle slope portion 222 formed of a gentle arc-shaped curve and a steep slope portion 223 that is extended from the gentle slope portion 222 to be close to the rotating shaft 101, to thus form a steep slope. Further, the cam cable winding grooves on which the cam cables 150 are wound are formed on the outer circumferential surfaces of the gentle slope portion 222 and the steep slope portion 223. Accordingly, the cam module 220 is rotated along with the pulley 110 when the bowstring 140 is pulled, and thus the cam cables 150 are sequentially wound on the gentle slope portion 222 and the steep slope portion 223 of the cam module 220 adjacent to the cam cable support portion 210. In addition, the cam 200 further includes a fixing unit that makes the cam module 220 rotated by a predetermined angle with respect to the pivot shaft 221 in order to control the draw length of the let-off state of the compound bow, and that makes the cam module 220 fixed to the pulley 110 at a position where the cam module 220 has been rotated.
The fixing unit makes the cam module 220 fixed to the pulley 110 at a position where the cam module 220 has been rotated. To this end, an arc-shaped positioning hole 230 centered at the pivot shaft 221 is formed in the cam module 220, and a coupling hole (not shown) is formed in the pulley 110 to which the cam module 220 is coupled. Thus, when the cam module 220 is rotated by a predetermined angle around the pivot shaft 221, and a coupling member 115 is coupled to the coupling hole (not shown) formed in the pulley 110 at a predetermined position of the positioning hole 230, the cam module 220 is coupled to the pulley 110. When a bolt as the coupling member 115 is inserted into the positioning hole 230 and is screw-coupled into the coupling hole (not shown) formed in the pulley 110, the cam module 220 is coupled to the pulley 110. In addition, a position display part indicated by numbers are provided around the positioning hole 230, in order to display position at which the cam module 220 is fixed. The cam modules 220 that are respectively coupled to the upper and lower pulley assemblies should have an identical rotational angle, and thus the position display part is indicated by numbers so as to see the rotational angle of each cam module 220. In addition, an arc-shaped coupling hole 116 centered at the pivot shaft 221 is additionally formed in the pulley 110, and a bolt 225 is coupled to a bolt hole formed at a predetermined position of the cam module 220.
When considering a lower pulley 110 in order to adjust a draw length of a bowstring 140 at a let-off state in a compound bow 100 illustrated in FIGS. 1 to 3, a bolt 115 engaged in a position adjusting hole 230 of a cam module 220 is removed, and then the cam module 220 is rotated by a predetermined angle around a pivot shaft 221 formed at a position spaced from a rotating shaft 101 of the lower pulley 110 to obtain a user desired draw length of the bowstring 140 at a let-off state. Then, the cam module 220 is fixed to the lower pulley 110 by using the bolt 115 again at the rotated position of the cam module 220, as shown in FIG. 3. In this case, the cam modules 220 respectively coupled to the lower and upper pulleys 110 are moved at an identical rotation angle.
When the cam module 220 is rotated around the pivot shaft 221 by a predetermined angle from the cam cable support portion 210, and the cam module 220 is fixed again to the lower pulley 110 at the rotated position of the cam module 220, as shown in FIG. 3, a winding length of a cam cable 150 wound on a smooth curved portion 222 of the cam module 220 is increased in comparison with FIG. 2. The length of the cam cable 150 wound on the cam module 220 from the cam cable support portion 210 up to the let-off state is eventually increased, to thereby increase a draw length of the bowstring 140.
By the way, in such a conventional compound bow, the cam modules 220 on which the cam cables 150 are wound are respectively coupled to an equi-directional side of the lower and upper pulleys 110, and rotating wheels 120 are respectively mounted more spaced apart from the lower and upper pulleys 110 than the cam modules 220. It is a factor of enlarging distortion of bow limbs 103 and also lowering accuracy of an arrow when the bowstring 140 is pulled that a configuration of the rotating wheels 120 that are respectively coupled to the pulleys 110 is formed higher than a configuration of the cam modules 220 that are respectively coupled to the pulleys 110.